The present invention relates to aluminum brazing alloys used for brazing aluminum structural members or articles and more particularly to alloys suitable for fabricating plate fin heat exchangers adapted for superhigh pressure service by brazing.
Conventionally, plate fin heat exchangers made of aluminum have been fabricated by means of an appropriate brazing technique such as dip brazing, vacuum brazing or atmospheric brazing wherein aluminum brazing alloys containing 4.5 to 13.5% Si and optionally, the addition of less than 3% Mg or 2.3 to 4.7% Cu and 9.3 to 10.7% Zn have been commonly employed. The above-mentioned silicon containing brazing alloys with or without the optional elements may further contain Be and Bi with a view to improving wettability. Throughout the present specification, percentages are by weight unless otherwise indicated. Besides the foregoing brazing alloys, aluminum alloys containing 4.5 to 13.5% of Si, less than 3% of Mg and 0.1 to 1.5% of Sr is proposed as aluminum brazing alloys capable of achieving high strength brazed joints in the fabrication of large-sized heat exchangers in Japanese patent application laid-open No. 56-169 744.
Superhigh pressure service plate fin heat exchangers are specified by the rupture pressure when rupture of the heat exchanges is caused due to the internal pressure. Strength at brazed joint portions is regarded as a dominant factor of the rupture pressure and the strength mainly depends on the width of the fillet and the structure in the brazed joint portions. In the fabrication of large scale heat exchangers, since the width of the fillet is greatly restricted by a prolonged preheating time for brazing and other brazing conditions, a satisfactory improvement in brazing can not be attained and the rupture pressure remains at a low level.
On the other hand, in general, the metallugical structure of the brazed portion can be refined by increasing the cooling rate after brazing, whereby the strength can be increased. However, practically, it is very difficult to increase the cooling rate in the construction of a large-sized heat exchanger.
For the foregoing reasons, applicable pressure in heat exchangers practically used is at most in the order of 50 kg/cm.sup.2.
Further, the aluminum brazing alloy with Sr, addition as set forth above, is inferior in wettability, whereby causing cavities to develop in brazed portions.